After the strategy is established, the effect of implied volatility depends on where the stock is relative to your strike prices.
If your forecast was correct and the stock price is approaching or below strike A, you want implied volatility to decrease. That’s because it will decrease the value of the near-the-money option you sold faster than the in-the-money option you bought, thereby increasing the overall value of the spread.
If your forecast was incorrect and the stock price is approaching or above strike B, you want implied volatility to increase for two reasons. First, it will increase the value of the out-of-the-money option you bought faster than the near-the-money option you sold, there by increasing the overall value of the spread. Second, it reflects an increased probability of a price swing (which will hopefully be to the downside).